The major cell-cell junction of the adult kidney podocyte is the slit diaphragm, an important component of the kidney filtration barrier. The loss of the slit diaphragm during foot process effacement results in proteinuria and contributes to the progression of many chronic kidney diseases such as diabetes and HIV-associated nephropathy. The developmental formation and adult homeostasis of podocyte cell junctions are processes that are not fully understood, but are important for the development of therapeutic strategies to repair or replace podocytes that are damage or lost in disease. We have recently identified the claudin-like cell junction protein TM4SF10 and the Fyn binding protein ADAP as podocyte-expressed proteins that affect lamellipodia formation, Fyn activity, and Nephrin phosphorylation. TM4SF10 is transiently expressed during podocyte development and is re-expressed during injury repair. Thus, we hypothesize that in disease, TM4SF10 may function to protect the podocyte during foot process effacement by preserving a simplified cell junction and by modifying the activities of Fyn. We also hypothesize that ADAP, a large adapter protein with known roles in integrin signaling, functions as a scaffolding center in the podocyte foot process integrating actin cytoskeleton dynamics at both the slit diaphragm and in integrin attachments of the foot process. We will test this hypothesis by determining the association of ADAP binding to TM4SF10 and other key intracellular components of the slit diaphragm complex using standard mutational analysis and in vitro binding assays, and assess functional effects based on lamellipodia extension, integrin binding, and Nephrin phosphorylation. Proposed studies also will include further characterize of the renal phenotype in ADAP knockout mice and further development and characterization of a knockout mouse model for TM4SF10. Podocyte cells lines from these mice will be established to investigate the requisite functions of TM4SF10 and ADAP in podocytes with regard to regulating integrin cell attachments and Nephrin phosphorylation. In addition, we have established TM4SF10 functions to directly suppress Fyn activity, possibly by sequestering Fyn away from the plasma membrane. This will be investigated with real time imaging and co-localization methods assessing intracellular trafficking of Fyn in the presence and absence of TM4SF10. These studies describe the first integrated molecular mechanism connecting cell-cell adhesion at the slit diaphragm with cell-matrix adhesion with integrin binding in the foot process, events that require synchronization during foot process formation and retraction. These studies will have important implications toward the development of novel therapies directed at glomerular diseases characterized by foot processes effacement where preventing or repairing damaged podocytes would be beneficial.
Lay Summary to treat diseases that damage the kidney's filtering apparatus (glomerulus) requires the development of strategies that encourage regeneration of new kidney cells. This requires a fundamental understanding of how the specialized cells of the kidney renew and form their unique structure. Important in the glomerulus repair process is how cell-cell junctions are established and maintained between podocytes, as these cell junctions are a key component of the blood filtering apparatus. These studies will provide important information on how the podocyte forms during development, and how it repairs itself in response to damage caused by disease.
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